Separately, French Prime Minister Edouard Philippe said four bodies have been recovered in Saint Martin after Irma clobbered the Caribbean island, while none had been found on the nearby isle of Saint Barthelemy.

Mr Philippe said that the death toll was provisional and could still change, but he appeared to be revising lower an earlier toll of eight victims announced by the interior minister earlier in the day.

The most potent Atlantic Ocean hurricane ever, Irma weakened only slightly Thursday morning and remains a powerful Category 5 storm with winds of 285 kilometres per hour, according to the US National Hurricane Center.

The storm was increasingly likely to rip into heavily populated South Florida early Sunday, prompting the governor to declare an emergency and officials to impose mandatory evacuation orders for parts of the Miami metro area and the Florida Keys.

"This could easily be the most costly storm in US history, which is saying a lot considering what just happened two weeks ago," said Brian McNoldy, a hurricane researcher at the University of Miami, alluding to the damage caused by Hurricane Harvey.

More than half the island of Puerto Rico was without power, leaving 900,000 in the dark and nearly 50,000 without water, the US territory's emergency management agency said in the midst of the storm. Fourteen hospitals were using generators after losing power, and trees and light poles were strewn across roads.

President Donald Trump approved an emergency declaration for the US Virgin Islands and Puerto Rico, allowing the Federal Emergency Management Agency and other agencies to remove debris and give other services that will largely be paid for by the US government.